WANTED: Libya's Moammar Khadafy, who is in town for the UN meeting, struts about with immunity, but could be busted if a DA gives the word.AP

TOMORROW brings excruciating embarrassment for the United Nations.

It will honor the worst man left in the world, who now devotes his time to thwarting UN attempts to bring other international criminals to justice.

Moammar Khadafy will make a triumphant address to the assembled dignitaries (including a humiliated President Obama) unless a district attorney in New York arrests him for murder or torture or conspiracy to cause explosions or for any of the various crimes against humanity committed during 35 of his 40 years of dictatorship.

Khadafy, in New York today for a pre-speech summit, gets away with murder because European nations, and the corporations that influence their governments, are desperate to share in his oil wealth, and because he buys off the relatives of his victims with “blood money” ($2.7 billion for Lockerbie, $1 million per family for a UTA passenger jet, and further millions for US victims of his supply of Semtex to the IRA), accompanied by insincere apologies.

In Africa, his impunity is attractive to other corrupt or brutal rulers. In February, he was elected head of the African Union and he has transformed this organization into the main foe of the International Criminal Court, guaranteeing to protect Gen. Omar Bashir from its arrest warrant over his alleged crimes in Darfur.

In the past, Khadafy has ordered many assassinations of dissidents (“stray dogs”) and sponsored many terrorist groups, ranging from Baader Meinhof to Abu Nidal, while his “charity” provides lavish compensation to the families of Palestinian suicide bombers.

The legal excuse for his untouchability is sovereign (or head of state) immunity, the Machiavellian doctrine that for centuries protected political and military leaders from any kind of accountability other than by forcible overthrow.

But immunity is not what it used to be: the Pinochet decision by Britain’s highest court held that ex-dictators could be liable for torturing their own people, and then the International Court of Justice held that courts set up by the UN could prosecute government ministers for mass murder.

In due course, Serbian leader Slobodan Milosevic went to trial, followed by Charles Taylor, after the UN’s court in Sierra Leone had upheld the issue of an arrest warrant at a time when he still headed Liberia.

The particulars in this warrant, significantly, named Khadafy as an “unindicted co-conspirator” accusing him of sponsoring Taylor and Foday Sankoh, the brutal rebel leader whose “Operation No Living Thing” almost lived down to its name in Freetown, Sierra Leone.

The experienced prosecutor who obtained the Taylor warrant has publicly stated that he had the evidence to indict Khadafy. His successor, Stephen Rapp, has just left the Taylor trial to take up the post of ambassador for war-crimes prosecutions with the Obama administration.

If his replacement obtains an arrest warrant from the UN court, Khadafy would have no immunity if it were executed on him while in New York.

There are other ways for US law enforcement to collar him.

Britain gave the world the Pinochet precedent, but the United States provided the Manuel Noriega example when the Panamanian head of state was arrested, then convicted and jailed for exporting cocaine to the United States.

If Abdel Basset al-Megrahi was guilty of the Lockerbie bombing, then Khadafy must have given the order. Megrahi was a senior Libyan intelligence official, and there is no way that Khadafy’s intelligence services, run by his brother in-law, would commit an atrocity of this magnitude without his knowledge and approval.

This crime has such close connections to America, given the nationality of the airline and most of the victims, that a New York district attorney would have no difficulty claiming jurisdiction to arrest the man reasonably suspected of being an arch co-conspirator.

For now, however, Khadafy struts the world stage, a living embodiment of impunity.

Britain has been his leading appeaser: the Special Air Service trains his troops, Scotland Yard helps his police, and his dissidents have been arrested and jailed under the UK’s anti-terrorism laws.

Italy and France have welcomed him, and last month, the Swiss government issued a groveling apology for arresting his son, Hannibal, for brutally beating his servants.

So, over to America. Obama recently criticized Britain for pandering to Khadafy by encouraging Megrahi’s release.

This week, the United States has the opportunity to end Khadafy’s invulnerability, which derives not from his strength, but from the weakness of international law and those who have a duty to apply it.

Geoffrey Robertson, who has served part time as a UN appeals judge at its war-crimes court in Sierra Leone, is the author of “Crimes Against Humanity: The Struggle for Global Justice.”